1. Field of the Invention
This invention relates to a time measurement apparatus. In addition, this invention relates to a distance measurement apparatus. Furthermore, this invention relates to an apparatus for generating a clock signal or a set of different-phase clock signals (multi-phase clock signals) which can be used in, for example, a time measurement apparatus or a distance measurement apparatus.
2. Description of the Related Art
A prior-art distance measurement apparatus of a spread spectrum type which is mounted on an automotive vehicle measures the distance between the present vehicle and a preceding target object such as a preceding vehicle by using an electromagnetic wave modulated in accordance with a pseudo random noise code (for example, a maximum length code). Specifically, a beam of an electromagnetic wave whose amplitude is modulated in accordance with a pseudo random noise code of a predetermined bit length is emitted in a forward direction with respect to the body of the present vehicle. A moment of the transmission of the pseudo random noise code with the electromagnetic wave is memorized. The prior-art apparatus receives an echo beam caused by reflection of the forward electromagnetic-wave beam at a preceding target object. The received echo beam is converted into a binary echo electric signal. Calculation is made about the value of the correlation between the binary echo electric signal and the pseudo random noise code used for the modulation of the transmitted electromagnetic wave. A moment at which the calculated correlation value peaks is detected as a moment of the reception of the pseudo random noise code contained in the echo beam. The prior-art apparatus calculates the distance between the present vehicle and the preceding target object from the time interval between the moment of the transmission of the pseudo random noise code and the moment of the reception thereof, and also from the velocity of the electromagnetic wave.
In the prior-art apparatus, the pseudo random noise code has a sequence of bits, the number of which is predetermined. The correlation-value calculation is iterated at a period corresponding to one bit of the pseudo random noise code. Accordingly, the time interval between the moment of the transmission of the pseudo random noise code and the moment of the reception thereof is determined at a resolution corresponding to a 1-bit time interval (a 1-chip time interval). A resolution of the measured distance between the present vehicle and the preceding target depends on the resolution of the determination of the time interval. In the case where a clock frequency, the reciprocal of which corresponds to a 1-bit time interval, is equal to 20 MHz, the resolution of the measured distance between the present vehicle and the preceding target is equal to 7.5 m.
U.S. Pat. No. 6,218,982 B1 corresponding to Japanese patent application publication number 2000-121726 discloses a distance measurement apparatus in which a pseudo random noise code is generated synchronously with a reference clock signal. A first forward electromagnetic wave is transmitted in response to the pseudo random noise code. A first echo wave is received which is caused by reflection of the first forward electromagnetic wave at an object. The received first echo wave is converted into a binary signal. A value of a correlation between the binary signal and the pseudo random noise code is repetitively calculated at a predetermined period having a synchronous relation with the reference clock signal. A time interval taken by the first forward electromagnetic wave and the first echo wave to travel to and from the object is measured in response to a timing at which the calculated correlation value peaks. Then, a second forward electromagnetic wave is transmitted in response to a transmitted pulse signal. A second echo wave related to the second forward electromagnetic wave is received. The received second echo wave is converted into a received pulse signal. A delay circuit defers the transmitted pulse signal by a delay time corresponding to the measured time interval to generate a delayed transmitted pulse signal. A phase difference between the received pulse signal and the delayed transmitted pulse signal is measured at a resolution higher than a resolution corresponding to the predetermined period of the correlation-value calculation. A distance to the object is calculated on the basis of the measured time interval and the measured phase difference.
U.S. Pat. No. 5,477,196 corresponding to Japanese patent application publication number 7-183800 discloses a pulse signal device for encoding a pulse phase difference or controlling an oscillation frequency based on delayed signals sequentially outputted by a delay circuit. The encoding of a pulse phase difference or the oscillation frequency control can be simultaneously performed by using a single delay device. There is provided a frequency converter including a ring oscillator consisting of inverting circuits interconnected in the form of a ring, a pulse phase difference encoding circuit for encoding the cycle of a reference signal into a binary digital value based on a pulse outputted by the ring oscillator, an arithmetic circuit for multiplying or dividing the binary digital value by a predetermined value to generate control data, and a digitally controlled oscillation circuit for generating a pulse signal in a cycle in accordance with the control data. The ring oscillator is shared by the encoding circuit and the oscillation circuit. This makes constant the time resolutions of the encoding circuit and the oscillation circuit, thereby allowing accurate frequency conversion.
U.S. Pat. No. 4,559,606 corresponding to Japanese patent application publication number 60-51338 discloses an arrangement to provide an accurate time-of-arrival indication for a received signal. The arrangement comprises a plurality of correlation means coupled to the received signal. The received signal is clocked into each of the plurality of correlation means by a different one of a plurality of different phased clocks. The different phased clocks occur at a predetermined different time uniformly spaced over a given time interval. The arrangement further comprises integration means coupled to each of the plurality of correlation means to provide the indication of the time of arrival of the received signals. The plurality of correlation means includes a plurality of received-signal shift registers into which the received signals are shifted by a different one of the plurality of phased clocks, a common reference signal shift register and a plurality of comparison means each coupled to each stage of a different one of the plurality of received-signal shift registers and each stage of the common reference signal shift register to compare the contents of the associated stages of the plurality of received-signal shift registers and the common reference signal shift register and to produce an output signal indicative of the results of each of the comparisons.
Japanese patent application publication number 4-363687 discloses a distance measurement system having a transmitter side and a receiver side. In the transmitter side, a carrier signal outputted from a carrier oscillator is spread-spectrum-modulated in accordance with a PN (pseudo noise) code fed from a PN code generator. The transmitter side transmits the modulation-resultant spread spectrum signal as a forward radio wave. An echo radio wave caused by reflection of the forward radio wave at an object is received by the receiver side, being converted into a received spread spectrum signal thereby. The receiver side includes a variable delay circuit which variably shifts the phase of the PN code used in the transmitter side to get a phase-shifted PN code. The receiver side detects a correlation between the phase-shifted PN code and the received spread spectrum signal. The receiver side determines the quantity of the PN-code phase shift at which the detected correlation peaks. The distance to the object is calculated on the basis of the determined quantity of the PN-code phase shift.
Japanese patent application publication number 9-264949 discloses a random modulation radar apparatus including a transmitter for transmitting a forward electromagnetic wave of a random pulse form given by a maximum length code. The apparatus receives an echo electromagnetic wave caused by reflection of the forward electromagnetic wave at an object. The distance to the object is measured according to the propagation delay time from the moment of the transmission of the forward electromagnetic wave to the moment of the arrival of the echo electromagnetic wave. The phase of the maximum length code is shifted stepwise to get a phase-shifted code. The apparatus calculates the correlation between the phase-shifted code and a received signal which may indicate an echo electromagnetic wave. The apparatus detects a peak of the correlation, and decides the moment of the arrival of the echo electromagnetic wave according to the detection of the correlation peak. The apparatus includes a judgment device for determining whether the wave-transmitting action of the transmitter should be continued or terminated on the basis of the detected peak of the correlation. When a high S/N ratio is attained regarding the detection of the distance to an object, the wave-transmitting action of the transmitter in one distance measurement process is terminated in response to the result of the determination by the judgment device. The high S/N ratio occurs in the case where the distance to the object is relatively short.
Japanese patent application publication number 5-312950 discloses a distance measurement apparatus which includes a maximum length code generator. A laser diode generates laser light modulated in accordance with a maximum length code produced by the maximum length code generator. A beam of the laser light is emitted toward an object. The apparatus includes a phase shifter which shifts the maximum length code by different values within a time interval corresponding to one bit of the maximum length code. The phase shifter outputs a plurality of shifted maximum length codes. The apparatus includes a photodiode which receives an echo beam caused by the reflection of the forward laser beam at the object. The photodiode converts the received echo beam into a corresponding echo electric signal. The values of the correlations between the echo electric signal and the shifted maximum length codes are calculated. Two straight lines are determined on the basis of the calculated correlation values. A point of intersection between the two straight lines is calculated. The distance to the object is computed on the basis of the calculated intersection point. The measurement gain is adjusted in response to the strength of the echo electric signal.
Japanese patent application publication number 6-326574 discloses a delay control device which includes a delay circuit, a phase difference detection circuit, and a phase difference voltage conversion circuit. The phase difference detection circuit detects the phase difference between a signal inputted into the delay circuit and a signal outputted therefrom. The phase difference voltage conversion circuit is informed of the detected phase difference. The phase difference voltage conversion circuit generates a voltage depending on the detected phase difference. The generated voltage is applied to the delay circuit as a delay control voltage. The signal delay provided by the delay circuit is controlled in response to the delay control signal.
Japanese patent application publication number 8-46554 discloses a delay equalization circuit including an analog-to-digital converter which changes an analog signal into a corresponding digital signal. A shift register having taps delays the digital signal by different delay times to generate different delay-resultant signals. The delay-resultant signals are applied to input terminals of a matrix switch, respectively. The matrix switch has output terminals which can be connected with the input terminals thereof. The matrix switch can change connection between the input terminals and the output terminals. Delay-resultant signals which appear at the output terminals of the matrix switch are applied to fast Fourier transform devices, respectively. Each of the fast Fourier transform devices extracts xe2x80x9cnxe2x80x9d components of the associated delay-resultant signal which have xe2x80x9cnxe2x80x9d specified frequencies respectively. The xe2x80x9cnxe2x80x9d frequency components are applied to an inverse Fourier transform device. The inverse Fourier transform device subjects the applied frequency components to inverse Fourier transform to generate time-domain digital data. A digital-to-analog converter changes the digital data into a corresponding analog signal.
U.S. Pat. No. 5,789,985 corresponding to Japanese patent application publication number 8-265111 discloses a frequency multiplying device which multiplies the frequency of an externally-supplied reference signal PREF. The frequency multiplying device includes a digitally controlled oscillation circuit, a counter/data-latch circuit, and a control circuit. The digitally controlled oscillation circuit includes a ring oscillator formed of thirty-two inverting circuits in a ring configuration which are adapted to generate sixteen clock signals having a period that is thirty-two times the inversion time of each inverting circuit and a phase interval that is twice the inverting circuit inversion time, and produces an output signal POUT having a period that corresponds to frequency control data CD at a resolution of the phase difference time of the clock signals. The counter/data-latch circuit counts the clock signal RCK released by the ring oscillator within one period of the reference signal PREF and delivers the frequency control data CD of the count value to the digitally controlled oscillation circuit. The control circuit controls the operation of the digitally controlled oscillation circuit and the counter/data-latch circuit so that the oscillation output signal POUT having the frequency of the reference signal PREF multiplied by sixteenth (32/2) is generated by the digitally controlled oscillation circuit.
It is a first object of this invention to provide a time measurement apparatus having a high resolution.
It is a second object of this invention to provide a distance measurement apparatus having a high resolution.
It is a third object of this invention to provide an improved apparatus for generating a clock signal or a set of different-phase clock signals (multi-phase clock signals).
A first aspect of this invention provides a shift clock signal generating apparatus for generating a shift clock signal having a prescribed phase difference from a reference clock signal. The apparatus comprises a delay line receiving the reference clock signal and including a plurality of unit delay elements connected in cascade, wherein each of the unit delay elements provides a prescribed signal delay time, and the reference clock signal propagates in the delay line while being successively delayed by the unit delay elements; a shift clock signal output path; a group of switches having first ends connected with output terminals of the unit delay elements respectively, and second ends connected with the shift clock signal output path, wherein when specified one among the switches is in its on position, a delayed clock signal which results from delaying the reference clock signal by a prescribed time interval is transmitted via the specified switch to the shift clock signal output path as the shift clock signal; and switch controlling means for determining the specified one among the switches on the basis of data representing a phase difference of the shift clock signal from the reference clock signal, and for setting the specified switch in its on position.
A second aspect of this invention is based on the first aspect thereof, and provides a shift clock signal generating apparatus comprising a plurality of delay lines each being equal in structure to the previously-mentioned delay line, a plurality of groups of switches each being equal in structure to the previously-mentioned group of switches, a plurality of shift clock signal output paths each being equal in structure to the previously-mentioned shift clock signal output path, and a plurality of switch controlling means each being equal in structure to the previously-mentioned switch controlling means to generate shift clock signals having prescribed phase differences from the reference clock signal, the prescribed phase differences being different from each other, wherein a number of the delay lines, a number of the groups of switches, a number of the shift clock signal output paths, and a number of the switch controlling means correspond to a number of the shift clock signals.
A third aspect of this invention is based on the first aspect thereof, and provides a shift clock signal generating apparatus comprising a plurality of groups of switches each being equal in structure to the previously-mentioned group of switches, a plurality of shift clock signal output paths each being equal in structure to the previously-mentioned shift clock signal output path, and a plurality of switch controlling means each being equal in structure to the previously-mentioned switch controlling means to generate shift clock signals having prescribed phase differences from the reference clock signal, the prescribed phase differences being different from each other, wherein first ends of the switches in each of the groups are connected with the output terminals of the unit delay elements respectively, and wherein a number of the groups of switches, a number of the shift clock signal output paths, and a number of the switch controlling means correspond to a number of the shift clock signals.
A fourth aspect of this invention is based on the third aspect thereof, and provides a shift clock signal generating apparatus wherein the switch groups are connected with the output terminals of ones among the unit delay elements in correspondence with the prescribed phase differences of the related shift clock signals from the reference clock signal.
A fifth aspect of this invention is based on the fourth aspect thereof, and provides a shift clock signal generating apparatus wherein the unit delay elements are separated into groups having a number equal to a number of the shift clock signals, and the switch groups are connected with unit delay elements in the corresponding unit-delay-element groups respectively.
A sixth aspect of this invention is based on the first aspect thereof, and provides a shift clock signal generating apparatus further comprising a reference clock signal output path for outputting the reference clock signal to an external as it is.
A seventh aspect of this invention is based on the first aspect thereof, and provides a shift clock signal generating apparatus wherein each of the unit delay elements includes a gate circuit for providing the prescribed signal delay time.
An eighth aspect of this invention is based on the first aspect thereof, and provides a shift clock signal generating apparatus wherein the switch controlling means operates for determining the specified one among the switches on the basis of period data and ratio data, the period data representing a numeric value of a period of the reference clock signal while a time resolution is given by the prescribed signal delay time provided by each of the unit delay elements, the ratio data representing a ratio between a delay time of the shift clock signal relative to the reference clock signal and the period of the reference clock signal.
A ninth aspect of this invention is based on the eighth aspect thereof, and provides a shift clock signal generating apparatus wherein the ratio represented by the ratio data is equal to y/(x+1), and xe2x80x9cxxe2x80x9d denotes a predetermined natural number and xe2x80x9cyxe2x80x9d denotes a natural number in a range of xe2x80x9c1xe2x80x9d to xe2x80x9cxxe2x80x9d.
A tenth aspect of this invention is based on the ninth aspect thereof, and provides a shift clock signal generating apparatus further comprising a ring delay line including a plurality of unit delay elements connected in a closed loop and being equal in characteristics to the unit delay elements in the previously-mentioned delay line, wherein a pulse signal circulates through the ring delay line while being delayed by the unit delay elements; and time A/D converting means for counting a number of times the pulse signal goes round the ring delay line, for generating the period data in response to the counted number of times, and for feeding the period data to the switch controlling means.
An eleventh aspect of this invention is based on the eighth aspect thereof, and provides a shift clock signal generating apparatus further comprising a digitally controlled oscillation circuit for outputting a signal having a period controllable while a time resolution is given by the prescribed signal delay time provided by each of the unit delay elements, the digitally controlled oscillation circuit using control data in controlling the period of the signal outputted therefrom, the delay line receiving the signal outputted from the digitally controlled oscillation circuit as the reference clock signal, the switch controlling means operating for using the control data as the period data.
A twelfth aspect of this invention is based on the eighth aspect thereof, and provides a shift clock signal generating apparatus further comprising a digitally controlled oscillation circuit for outputting a signal having a period controllable while a time resolution is given by the prescribed signal delay time provided by each of the unit delay elements, the digitally controlled oscillation circuit using control data in controlling the period of the signal outputted therefrom, and a frequency divider circuit for dividing a frequency of the signal outputted from the digitally controlled oscillation circuit to generate the reference clock signal having a duty cycle of 50%, the delay line receiving the reference clock signal generated by the frequency divider circuit, the switch controlling means operating for doubling a period represented by the control data to calculate the period of the reference clock signal and for generating the period data in accordance with the calculated period.
A thirteenth aspect of this invention is based on the eleventh aspect thereof, and provides a shift clock signal generating apparatus wherein the digitally controlled oscillation circuit includes a ring delay line including a plurality of unit delay elements connected in a closed loop and being equal in characteristics to the unit delay elements in the previously-mentioned delay line, wherein a pulse signal circulates through the ring delay line while being delayed by the unit delay elements; time A/D converting means for counting a number of times the pulse signal goes round the ring delay line, for generating the period data in response to the counted number of times, and for outputting the period data; dividing means for dividing a value of the period data outputted from the time A/D converting means by a preset number to generate the control data; and signal outputting means for comparing a value of the control data and a number of times the pulse signal passes through a unit delay element in the ring delay line, and for outputting a prescribed-pulsewidth signal each time the value of the control data and the number of times become equal to each other.
A fourteenth aspect of this invention provides a time measurement apparatus comprising a shift clock signal generating means for generating a plurality of shift clock signals in response to a reference clock signal, the shift clock signals having a period equal to a period of the reference clock signal, the shift clock signals having phases different from each other; signal inputting means for converting an input signal into binary signals in response to the shift clock signals generated by the shift clock signal generating means respectively, the input signal containing a pulse train of a pseudo random noise code; correlation calculating means for calculating correlations between the pseudo random noise code and the binary signals generated by the signal inputting means; detecting means for detecting a moment at which a peak of the correlations calculated by the correlation calculating means occurs; and measuring a time interval from a prescribed moment to a moment of occurrence of the pulse train in the input signal on the basis of the moment detected by the detecting means.
A fifteenth aspect of this invention is based on the fourteenth aspect thereof, and provides a time measurement apparatus wherein a number of the shift clock signals is equal to a predetermined integer xe2x80x9cnxe2x80x9d, and differences between the phases of the shift clock signals are equal to the reference-clock-signal period divided by the predetermined integer xe2x80x9cnxe2x80x9d.
A sixteenth aspect of this invention is based on the fourteenth aspect thereof, and provides a time measurement apparatus wherein the correlations calculated by the correlation calculating means are in pairs each having correlations related to two among the shift clock signals which have phases most different from each other, and further comprising averaging means for averaging correlations in each of the pairs into a mean correlation, the detecting means operating for detecting a moment at which a peak of the mean correlations generated by the averaging means occurs as an indication of the moment of occurrence of the pulse train in the input signal.
A seventeenth aspect of this invention is based on the fourteenth aspect thereof, and provides a time measurement apparatus wherein the correlation calculating means includes synchronizing means for sampling the binary signals generated by the signal inputting means into second binary signals in response to the reference clock signal, and means for calculating correlations between the pseudo random noise code and the second binary signals, the correlation calculating means and the detecting means operating in response to the reference clock signal.
An eighteenth aspect of this invention is based on the seventeenth aspect thereof, and provides a time measurement apparatus wherein the detecting means operates for detecting a moment at which a peak of the correlations calculated by the correlation calculating means occurs on the basis of one of (1) a moment at which one of the correlations exceeds a preset threshold value and (2) a phase difference between the reference clock signal and one among the shift clock signals which corresponds to the correlation exceeding the preset threshold value.
A nineteenth aspect of this invention is based on the eighteenth aspect thereof, and provides a time measurement apparatus wherein the detecting means operates for detecting a moment at which a peak of the correlations calculated by the correlation calculating means occurs on the basis of a moment at which one of the correlations first exceeds a preset threshold value.
A twentieth aspect of this invention provides a spread-spectrum distance measurement apparatus comprising pulse train generating means for generating a pulse train of a pseudo random noise code in synchronism with a reference clock signal, the pseudo random noise code having a prescribed bit length; transmitting means for transmitting an electromagnetic wave modulated in accordance with the pulse train generated by the pulse train generating means; receiving means for receiving an echo wave caused by reflection of the electromagnetic wave transmitted by the transmitting means at an object to be measured, and for converting the received echo wave into a received pulse train signal; time measuring means for measuring a time interval from a moment of transmission of the electromagnetic wave from the transmitting means to a moment of reception of the echo wave by the receiving means on the basis of the pseudo random noise code and the received pulse train signal; and distance calculating means for calculating a distance to the object on the basis of the time interval measured by the time measuring means. The time measuring means comprises the time measurement apparatus of the fourteenth aspect of this invention.
A twenty-first aspect of this invention is based on the twentieth aspect thereof, and provides a spread-spectrum distance measurement apparatus wherein the pulse train generating means operates for generating the pulse train of the pseudo random noise code a plurality of times, and the time measuring means operates for measuring the time interval a plurality of times, and the distance calculating means operates for averaging the time intervals measured by the time measuring means into a mean time interval, and for calculating the distance to the object on the basis of the mean time interval.
A twenty-second aspect of this invention is based on the twenty-first aspect thereof, and provides a spread-spectrum distance measurement apparatus wherein the time measuring means operates for determining a center among the time intervals measured by the time measuring means, for excluding one or more among the time intervals which deviate from the center by greater than a prescribed value to get remaining time intervals, and for averaging the remaining time intervals into the mean time interval.